[R] Count missing variables in dataset

[Jan Sabee]

I have a problem about how to count missing variables in dataset.
I have a question for my customer. They are must choice one of answer
for example: A, B, C, D and E.
Now, I have dataset which is the result from my question, for instance:

x1 x2 x3 x4 x5
A  A  A   B  A
C  A  A   A  A
B  B  A   B  A
B  B  B   C  A
A  B  B   B  B
C  B  B   B  A

I know that my customer only choice A, B and C.
Are there any way to count variables that are not include in dataset.
I mean D and E is missing in dataset.

Thanks for your help.
Jan Sabee